I like Johnny Cash a lot. I like everything he does really.
~Bob Dylan (to Nat Hentoff – Autumn 1965)

In the end, Nashville Skyline is a lovely album but not a heavyweight contender, though its effects were major ones. Country music was despised, hick music when Dylan took it up. People were divided into the hip and the non-hip. The counterculture was in full swing and riddled with its own self-importance and snobbery. Nashville Skyline was a hard pill to swallow: but it did ’em good.
~Michael Gray (Bob Dylan Encyclopedia)

The 5th recording session for ‘Nashville Skyline’ took place on February 18, 1969. One master versions emerged.. the lovely “Girl from the North Country”. Johnny Cash shared vocal on all 38 takes…this is a highly bootlegged sessions… and people have uploaded most of it on youtube…

picture of Johnny Cash & Bob Dylan.. NOT from the studio sessions

Some background from wikipedia:

Nashville Skyline is the ninth studio album by American singer-songwriter Bob Dylan, released in April 1969 by Columbia Records.

Building on the rustic style he experimented with on John Wesley Harding, Nashville Skyline displayed a complete immersion into country music. Along with the more basic lyrical themes, simple songwriting structures, and charming domestic feel, it introduced audiences to a radically new singing voice from Dylan—a soft, affected country croon.

The result received a generally positive reaction from critics, and was a commercial success. Reaching number 3 in the US, the album also scored Dylan his fourth UK number 1 album.

The session on February 18 was devoted exclusively to duet covers with Cash. “One Too Many Mornings” and “I Still Miss Someone” were revisited, and rejected, yet again. “Matchbox”, “That’s All Right Mama”, “Mystery Train”, “Big River”, “I Walk the Line”, and “Guess Things Happen That Way”, all made famous by celebrated Sun recordings performed by Carl Perkins, Elvis Presley, and Cash himself, were all attempted on February 18, but none of these were deemed usable. Covers of Jimmie Rodgers’ “Blue Yodel #1” and “#5”, Cash’s “Ring of Fire” (written by his wife, June Carter and Merle Kilgore), “You Are My Sunshine”, “Good Old Mountain Dew”, the traditional ballad “Careless Love”, the traditional hymn “Just a Closer Walk with Thee”, “Five feet high and rising”, and “Wanted Man” (a song written by Dylan specifically for Cash) were also attempted, and all were rejected. There was little enthusiasm for any of these tracks, but one duet of Dylan’s, “Girl from the North Country” (which originally appeared on The Freewheelin’ Bob Dylan), was ultimately sequenced as the album’s opener.